The SELCAL system consists of a ground-based SELCAL encoder and a VHF or HF communication system and the airborne decoder. Each aircraft's "SELCAL decoder" is assigned a unique code. The airborne SELCAL decoder is connected to the aircraft's VHF and HF radios. When the aircraft is called, an aural alert is received and the active radio is identified on the cockpit annunciator panel, signaling the flight crew to monitor the company radio frequency.

The SELCAL is used for communications. The transponder is used to identify the aircraft on radar, and provide some information (depending on the mode of the transponder). Whereas the an aircraft will have a distinct SELCAL code, the transponder is a device where you select the code (usually on instruction from ATS, but there are some specific reserved codes, such as for emergencies, and flights outside controlled airspace). The transponder code is made up of 4 digits, each 0-7 (giving a total of 4096 distinct codes).

V/F

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